1. Technical Field
This invention relates to cordless telephones and more particularly to an apparatus and method employed in a cordless telephone for initiating synchronous frequency hopping for communicating in a frequency hopping system.
2. Description of the Prior Art
Recent rulings promulgated by the Federal Communication Commission (FCC) in the utilization of spread spectrum systems, including a frequency hopping system, now allow for increased spectral utilization for cordless telephones. Frequency hopping systems spread theft energy by changing, or hopping, the center frequency of the transmission many times a second in accordance with a pseudo-randomly generated list of communication channels. The result is a significantly higher signal-to-noise ratio than may be achieved by conventional techniques, such as amplitude modulation which uses no bandwidth spreading.
Cordless telephones designed to operate in a frequency hopping system have now become available. One example of such a cordless telephone has been designed in accordance with the teachings of U.S. Pat. No. 5,323,447 which issued on Jun. 21, 1994 and also U.S. Pat. No. 5,353,341, which issued on Oct. 4, 1994. Both the patent and patent application are incorporated herein by reference.
The cordless telephone disclosed in U.S. Pat. No. 5,353,341 conserves battery power in a handset unit when this unit is located remote from an associated base unit and configured in a standby state. While configured in the standby state, the handset unit resides in a low-power monitoring mode which includes deactivating certain internal circuitry and reducing the on-time state of other internal circuitry for conserving battery power.
While configured in the standby state, the cordless telephone must be able to quickly respond to the base unit and also initialize its frequency hopping with the base unit. This involves reconfiguring the handset unit from the standby state to a full ON operating state and also having both units execute an initialization process during which the frequency hopping between these units is synchronized. Should ongoing communications between the base unit and the handset unit be inadvertently interrupted, the initialization process is also executed at this time.
The initialization process disclosed in U.S. Pat. No. 5,353,341 includes performing in order: 1) the base unit sending one in a series of start hopping messages and the handset unit receiving this message; 2) the handset unit replying to the base unit by sending one in a series of first acknowledge messages in response to receipt of the start hopping message; and 3) the base unit replying to the handset unit by sending one in a series of second acknowledge messages acknowledging receipt by the base unit of the first acknowledge message from the handset unit. Each message (start hopping message, first acknowledge message and second acknowledge message) from each of the series of three messages is transmitted in the designated sequential order from the base unit or the handset unit, as just described. Once the transmission of the series of messages is completed, synchronous frequency hopping between the handset unit and base unit begins. In obtaining synchronization by this process, a delay, typically six to eight transmission frames or time periods, in both the base unit and the handset unit is incurred before the appropriate acknowledge signal is received. Such delay is incurred each time the base unit sends a start hopping message from the series of start hopping messages. Thus, the processing time in the handset unit and base unit is not fully optimal, since each has to wait for receipt of the acknowledge signal from the other unit. Additionally, this increases the time for the two units to begin frequency hopping.
Although the described initialization process is effective in establishing and reestablishing communications between the base unit and the handset unit, it does not quickly obtain synchronization between the base unit and the handset unit. Fast synchronization is desirable to minimize the time encountered by a user of the cordless telephone while waiting for these units to establish communications or, as necessary, reestablish communications.